I've got both the 85/1.8 and 100/2.0.

Really, they're identical twins, both about the same size, weight, and all that.
85/1.8 is wildly more popular than the 100/2.0, so if you're buying second-hand then you're way more likely to find one, and find it cheaper.
Buying new, the 100/2.0 is just a bit more expensive. It's also 15mm longer focal length, and the 1.8 vs 2.0 you won't notice. And it's also a tiny bit sharper on test charts, but I haven't noticed in real life.

Frankly, I couldn't deicde which to get, so I got both within a few months of each other, and I use both on crop (7D) and FF (3) regularly depending on where i'm going and what i'm doing.
If you still can't decide, try thinking about what 'L' lens you're more likely to want/get later down the track, 100/2.8 L macro or 85/1.2 L, then buy the other focal length in the 1.8 or 2.0 aperture so you won't double up.
Or just get both.